For twenty-five years, automation has been playing a continually bigger part in avionics, both civil and military, so that crews are required to make ever more use of electronic systems, and have ever less direct influence on the aircraft's primary flight controls. This trend has been further accentuated over the last fifteen years with the generalization in aircraft of the flight management systems, commonly designated by the term FMS (for Flight Management System).
A flight management system consists of various functional components which allow the crew to programme a flight on the basis of a navigation database. The crew provides it with a flight plan, which consists of a set of waypoints linked by segments or “legs”. The system calculates a lateral and vertical trajectory making it possible to follow the flight plan, on the basis of the characteristics of the aeroplane, the data provided by the crew and the environment of the system. The positioning and guidance functions then collaborate to aid the aircraft to remain on this trajectory.
Functions for interfacing with the crew and with the ground allow the human to intervene in the navigation loop since he alone is responsible for the progress of the flight.
It frequently happens that an aircraft deviates temporarily from its flight plan, for example on instruction from an air traffic controller during an approach phase before landing. To return to their flight plan, the crew of the aircraft must define the new active reference, that is to say for example the new current leg, called the “active leg”, or the new point from which the aircraft comes, called the “From waypoint”, of the flight plan and then update the flight plan so as to render it consistent and so that the flight management system can recalculate a trajectory. The term “active reference” is known and designates the current reference used for the guidance of the aircraft.
When a member of the crew wishes to designate a new active reference to the flight management system, he has two tools at his disposal:                the direct rejoining of a waypoint, designated in the flight plan, with the aid of a function of the “Direct to” type, as it is known;        the erasure of the From waypoint.        
In the first case, he modifies the structure of his flight plan by creating a new leg between the position of his aeroplane and the selected point.
In the second case, he must perform multiple manipulations for erasing the origin point of the active leg. This operation is known as manual sequencing of the flight plan.
The most operationally acceptable procedure is to choose a radial interception towards the point that one wishes to home in on, while preserving the course of the original leg. But this operation is not very simple since it demands particular attention and it modifies the nature of the leg leading to the point that the operator wishes to home in on.
To summarize, currently, the modification of the flight plan and/or the rejoining of the latter when the aircraft has deviated therefrom requires the operator, knowing the capabilities of the flight management system, to undertake a cognitive effort or a significant workload in order to carry out this task of designating the active reference of the flight plan.
French patent application No. 0706471 may be cited as a method for aiding navigation aimed at facilitating the manual sequencing of a flight plan. This method in fact proposes a semi-automatic function for sequencing the flight plan. It applies in the case where an aircraft has deviated from its flight plan for any reason and when the crew wishes at a given moment to rejoin their original flight plan. According to this method, the flight management system must anticipate the trajectory for rejoining the flight plan and prompt the pilot by proposing the waypoint towards which the aircraft ought to steer. The pilot may or may not consent to this proposal. However, according to this invention, the sequencing of the flight plan is not carried out automatically by the flight management system.
So, no method exists today making it possible to relieve the operator of a significant workload demanding real cognitive effort when he wishes to reintegrate a flight plan, at the level of a waypoint or a leg of his choice. This operation is nevertheless frequently useful, notably when an aircraft deviates from its flight plan for any reason, for example when an air traffic controller asks it to divert during an airport approach phase.
An aim of the invention is thus to allow the crew of an aircraft to designate in an explicit manner the active reference of the flight plan, that is to say for example the leg or the origin point of the legthat they wish to see become active.